Radar antenna radome construction

ABSTRACT

A RADAR ANTENNA RADOME CONSTRUCTION DEFINING AN ELECTROMAGNETIC WINDOW FOR TRANSMITTING PRESELECTED MICROWAVE ELECTROMAGNETIC ENERGY. THE CONSTRUCTION IS DEFINED BY A WALL STRUCTURE HAVING AT LEAST THREE SPACED HIGH DIELECTRIC LAYERS, A LOW DIELECTRIC LAYER BETWEEN EACH PAIR OF HIGH DIELECTRIC LAYERS, AND A PLURALITY OF PARALLEL ELECTRICAL CONDUCTORS IN THE HIGH DIELECTRIC LAYERS. THE WALL STRUCTURE IS ARRANGED TO PROVIDE HIGH TRANSMISSION EFFICIENCY   OVER A WIDE RANGE OF INCIDENCE ANGLES, LOW REFLECTION OVER A WIDE RANGE OF INCIDENCE ANGLES, AND LOW PHASE DELAY VARIATION OVER A WIDE RANGE OF INCIDENCE ANGLES.

F61). 2, 1971 c, 0075 EIAL 3,560,986

RADAR ANTENNA RADOME CONSTRUCTION Filed Feb. 24, 1969 wfm/enfbrar lea nard C .f/oofs,

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United States Patent Oflice 3,560,986 Patented Feb. 2, 1971 3,560 986 RADAR ANTENNA RAD OME CONSTRUCTION Leonard C. Hoots and Dallas L. Addis, Marion, Va., assignors to Brunswick Corporation, a corporation of Delaware Filed Feb. 24, 1969, Ser. No. 801,404 Int. Cl. H01q N42 US. Cl. 343872 17 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention This invention relates to electromagnetic transmission devices and in particular to radome constructions.

Description of the prior art A number of different radome constructions have been developed for transmitting electromagnetic energy in the microwave spectrum. One such structure is shown in the US. patent to Oleesky et al. 3,002,190. In the structure of this patent, a plurality of low dielectric core layers are separated by thin high dielectric layers. There is no suggestion, however, of providing conductive material in any of the layers to provide a controlled impedance.

A similar structure provided in the prior art for absorbing electromagnetic energy reflected from a dense plasma is disclosed in Hollingsworth 3,295,131 wherein only three layers are provided. Hollingsworth arranges conductors in association with the wall structure to rotate electric fields entering the structure so that absorption may occur at the resistive surfaces thereof.

The prior art further contemplates utilizing a plurality of conductive sheet elements without intervening di electric materials, as in the Us. patent to Lerner 3,267,- 480, and teaches the use of spaced parallel conductors perpendicular to the adjacent layers, as in the US. patent to Wickersham, Jr. 2,921,312. Wickersham, Jr. further teaches, in his patent 3,089,142, a random arrangement of parallel conductor strips to effect a broadened response.

Thus, the prior art teaches a number of different electromagnetic energy transmitting wall structures made up of a number of layers of different materials.

SUMMARY OF THE INVENTION The present invention comprehends an improved electromagnetic energy transmitting wall structure providing a substantial improvement over the prior art structures in low reflection, high transmission, and low phase delay variation over a wide range of incidence angles of the electromagnetic radiation. The present invention comprehends an arrangement of an odd number of at least five layers of alternating high and low dielectric materials, with the high dielectric layers having spaced parallel conductors therein. The conductors are preferably spaced apart less than the wavelength to be transmitted, and the dielectric layers preferably have a thickness less than the wavelength. The embedded conductors therein are arranged to define equivalent lumped inductances and the dielectric layers are arranged to define equivalent capacitances preselected to cause a resonant condition to obtain for the desired electromagnetic energy transmission frequency. The electric field is caused to be parallel to the embedded conductors while the angle of incidence may be allowed to vary over a wide range such as from zero to More specifically, the present invention comprehends a wall structure for transmitting microwave electromagnetic energy having a preselected wavelength, comprising at least three spaced high dielectric layers, a low dielectric layer between each pair of high dielectric layers, and a plurality of parallel electrical conductors in each of the high dielectric layers extending parallel to the flatwise extent of the layers.

Further more specifically, the present invention comprehends such a wall structure wherein the conductors are spaced apart in each high dielectric layer approximately one-half the preselected wavelength. Still more specifically, the present invention comprehends such a wall structure wherein the conductors have a width in the direction of their spacing in the high dielectric layers substantially less than one-fourth the preselected wavelength.

Further more specifically, the invention comprehends such a wall structure wherein the capacitive means and the inductive means are preselected to be resonant at the frequency of the preselected wavelength microwave.

BRIEF DESCRIPTION OF THE DRAWING Other features and advantages of the invention will be apparent from the following description taken in onnection with the accomaanving drawin wherein:

FIG. 1 is a side elevation of an aircraft provided with a radome construction embodying the invention; and

FIG. 2 is a fragmentary perspective section of the wall structure thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the exemplary embodiment of the invention as disclosed in the drawing, a radome construction generally designated 10 is shown installed on an aircraft 11 for housing a conventional radar antenna (not shown) thereof. The radome 10 is defined herein by an improved wall structure generally designated 12 arranged to transmit preselected microwave electromagnetic energy with low reflection, high transmission, and low phase delay variation over a wide range of incidence angles. The wall structure 12 includes an odd number of alternating layers of high and low dielectric materials and in the illustrated embodiment of FIG. 2, comprises a five-layered structure including layers 13, 14 and 15 formed of high dielectric materials and layers 16 and 17 formed of low dielectric material. Each of the high dielectric layers 13, 14 and 15, is provided with a plurality of parallel electrical conductors 18 embedded therein and extending parallel to the flatwise extent of the layer, as best seen in FIG. 2.

The wall structure layers may comprise fiberglass laminates and illustratively, the high dielectric layers 13, 14 and 15 may have a dielectric constant of approximately 4.3 and the low dielectric layers 16 and 17 may have a dielectric constant of approximately 1.1. The low dielectric layers 16 and 17 have a thickness of approximately onefourth of the wavelength of the electromagnetic radiation to be transmitted through the radome wall structure. The high dielectric layers may have a thickness substantially less than the thickness of the low dielectric layers and in the illustrated embodiment have a thickness less than approximately one-tenth thereof. The conductors 18 may comprise any suitable electrical conductive elements and in the illustrated embodiment of FIG. 2 comprise wires. The conductors 18 are spaced apart less than approximately one-half the wavelength of the electromagnetic radiation to be transmitted and their diameter and width is less than approximately one-fourth. The embedded conductors effectively define equivalent lumped inductances whereas the dielectric layers effectively define equivalent capacitances. The inductances and capacitances are effectively parallel to provide a resonant condition which, by suitably controlling the parameters of the layers, may be preselected to occur at the desired electromagnetic radiation frequency. The wall structure 12 is arranged so that the electric field of the electromagnetic radiation is parallel to the conductors.

The thickness of the respective high dielectric layers may be similar and the thickness of the respective low dielectric layers may be similar. However, if desired, the middle high dielectric layers, such as layer 14, may be made somewhat different. Each of the high dielectric layers may have a thickness substantially less than one-fourth of the wavelength of the electromagnetic radiation to be transmitted therethrough.

Examples of such a wall structure 12 adapted for transmitting electromagnetic radiation at a frequency of 10 .8 gigacycles are illustrated in the following tables:

TABLE II.-COMPUTED PERFORMANCE AND SUSCEP- VALUES FOR EQUAL SUSCEPTANOE ARRANGE- l Insertion Susceptance Incidence Transmission, phase Reflection, in layers angle, percent delay, percent 1,3,5

A protective coating may be applied to the outer surface of the wall structure 12, such as protective coating 19 On the outer surface of layer 13, as shown in FIG. 2, for mechanical protection purposes.

The spacing of the conductors, or wires 18, is preselected as discussed above to be less than approximately one-half the wavelength of the electromagnetic radiation to be transmitted. The spacing may be varied somewhat to provide optimum matching of the high dielectric layer characteristics. Further, the inductance X produced by the spaced conductors 18 preferably should provide a correction factor F which is totally real in the following equation:

where: s=wire spacing d=wire diameter 20 1 :correction factor Z (sec 0) :transmission line characteristic impedance.

The inductance may further be slightly adjusted to provide optimum performance by adjusting the thickness of the high dielectric layers.

The functioning of wall structure 12 is extremely simple. The wall structure is intended to minimize reflection of the impinging electromagnetic radiation. Assuming that the radiation is directed downwardly through the wall structure 12, as shown in FIG. 2, to enter the wall surface of layer 13, a portion of the energy of the radiation is transmitted into the wall structure and a portion thereof is reflected from the outer surface of layer 13. A por- 3 tion of the transmitted radiation is reflected back outwardly from the inner surface of the layer 13 and a similar reflection condition exists at each of the surfaces of the other dielectric layers and the conductors of the wall structure. The total reflected radiation energy is the vector sum of each of these reflected portions. The magnitude of the reflections at the dielectric interfaces is dependent upon the ratio of the relative dielectric constants of the materials at the opposite side of each of the reflecting surfaces. In the case of the conductors, reflection depends upon the equivalent susceptance.

As a result, however, of the preselection of the diel ctric constants and the conductor size, shape, and distribution to define a resonant inductance-capacitance system, the transmission loss in wall structure 12 is effectively minimized permitting optimum transmission of the TABLE III.-COMPUTED PERFORMANCE AND SUSCEPTANCE VALUES FOR LOW REFLECTION AND HIGH TRANSMISSION ARRANGEMENT Insertion Susceptance in Transmission, phase Reflection, percent delay, percent Layers 1 & 5 Layerf'a Incidence angle 0 96.6 -l6.6 0 .425 .445 97. l 16. 7 0 42.5 445 97. J 1J. 1 0 42-1 491 97. 7 21.1 0 -.422 519 97. 5 22.2 0 418 522 16. -21. 5 0 411 .496 05. 9 16. 2 0 300 433 94. 0 8.4 U -.385 -.385 88.8 -l.6 .3 .376 -.370

TABLE IV.COMPUTED PERFORMANCE AND SUSCEPTANCE FOR LOW INSERTION PHASE DELAY ARRANGEMENT Insertion Susceptance in- Transmission, phase Reflection, percent delay, percent Layers 1 & .5 Layer 3 Incidence angle 0. 89. 3 0 8.3 425 006 10 88. 5 0 9. 0 425 006 20 86. 5 0 11. O 424 077 30 84. 1 0 13. 4 422 112 40.. 83.1 0 14. 2 41B 168 50.. 85. 3 0 11.8 -.411 231 60.. 8s). (1 0 6. 7 399 286 70.. 92. 3 0 2.1 385 331 88.7 0 .5 .370 370 preselected electromagnetic radiation through the wall structure with effectively minimum reflection over a wide range of incidence angles. Some transmission loss occurs in the wall structure as a result of the loss tangent of the materials. The loss tangent effectively defines a resistance in combination with the effective inductance and capacitance of the wall structure layers and, thus, may be readily considered in the design of the wall structure for the radiation wavelength to be transmitted.

While we have shown and described one embodiment of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1. A wall structure for transmitting microwave electromagnetic energy having a preselected free-space wavelength, said wall structure comprising:

at least three spaced high dielectric layers;

a low dielectric layer between each pair of high dielectric layers; and

a plurality of parallel electrical conductors in each of said high dielectric layers extending parallel to the fiatwise extent of said layers, dielectric layers defining capacitive means and said conductors therein defining inductive means.

2. The wall structure of claim 1 wherein the low dielectric layers have a thickness of approximately onefourth said preselected wavelength.

3. The wall structure of claim 1 wherein said conductors are spaced apart in each high dielectric layer less than approximately said preselected wavelength.

4. The wall structure of claim 1 wherein said conductors have a width in the direction of their spacing in said high dielectric layers substantially less than onefourth said preselected wavelength.

5. The wall structure of c aim 1 wherein said capacitive means and said inductive means are preselected to be resonant at the frequency of said preselected wavelength microwave.

6. The wall structure of claim 1 wherein each of said high dielectric layers has a thickness less than one-fourth said preselected wavelength.

7. The wall structure of claim 1 wherein the conductor configuration in each high dielectric layer is similar.

8. The wall structure of claim 1 wherein the thickness of each of the outermost high dielectric layers is similar.

9. The wall structure of claim 1 wherein the thickness of each high dielectric layer between said low dielectric layers is smaller than the thickness of each of the outermost high dielectric layers.

10. The wall structure of claim 1 wherein the dielectric characteristics of each of said high dielectric layers is similar.

11. The wall structure of claim 1 wherein the dielectric characterisctis of each of said low dielectric layers is similar.

12. The wall structure of claim 1 further including a protective coating on the outer surface of said Wall structure.

13. The wall structure of claim 1 wherein the thickness of said layers is preselected to provide effectively minimum reflection of said microwave over a substantial range of incidence angles.

14. The wall structure of claim 1 wherein the thickness of said layers is preselected to provide effectively maximum transmission of said microwave over a substantial range of incidence angles.

15. The wall structure of claim 1 wherein the thickness of said layers is preselected to provide effectively minimum phase delay of said microwave over a substantial range of incidence angles.

16. The wall structure of claim 1 wherein the number of layers is extended to inc ude more dielectric layers, such that the total number of layers is odd, the layers are alternating high and low dielectric, and each high dielectric layer is provided with a plurality of electrical conductors extending parallel to its flatwise extent.

17. The wall structure of claim 1 wherein the number of layers is extended to include more dielectric layers, such that the total number of layers is odd, the layers are alternating high and low dielectric, and each high dielectric layer is provided with a plurality of electrical conductors extending parallel to its flatwise extent, and each low dielectric layer is approximately one-fourth of said preselected wavelength.

References Cited FOREIGN PATENTS ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 

